Voice-operated switching circuit for two-way telephony



D. 2, 1941. D MITCHELL 2,267,622

VOICE-OPERATED SWITCHING CIRCUIT FOR TWO-WAY 'TELEPHONY Filed May 7, 1940 2 shams-sheet 1 FAST SUBS T4 T/ ON /N VEN TOR By D. M/TCHELL A from/EY VOICE-OPERATED SWITCHING CIRCUIT FOR TWO-WAY TELEPHONY Annu lllllAAlA n vuvIVVV A TTO/QNE V vPatented Dee. 423, 41941 VOHCE-OPBATED SWITCHING CIRCUIT FOB-TWO-WAY TELEPHONY i Deren Mitchell, Bound Brook, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 7, 1940, Serial No. 333,735 A Claims.

The present invention relates to the two-way transmission of speech or similar waves over a system which has, atleast at its terminals, separate transmitand receive circuits with-transmission control devicesoperated under control of the waves traversing the system.

The invention in certain of its aspects represents an improvement upon the system disclosed and claimed in my prior application Serial No. 255,223, iiled February 8, 1939, now Patent No. 2,213,991, Sept. 10, 1940. The systemthere disclosed comprises a two-way telephone system especially adapted for carrying on two-Way communication between distantly separated groups ol persons on a conference basis, and it involves certain types of voice control adapted to such a service.

The present invention is also adapted to telephone conference service and provides break-in circuits for enabling a talking party at any stastation or stations of the system merely by raising his voice and without the manipulation of push buttons or the like. These provisions are made with the aim of simulating conditions inan actual conference room where a person naturally raises his voice to gain the attention of the other persons in the group.

The invention also includes features to protect against false operation of the transmission control circuits by noise, and other improvement features that will appear as the description proceeds.

While the invention will be specically disclosed as embodied Vin a telephone conference Fig. 2 is a schematic circuit diagram of theA apparatus located at the west toll oilice shown in Flg.,1.

In Fig. 1 the upper and lower horizontal lines tion to gain the attention of parties at the other indicate speech transmission paths in opposite directions between the two substations. 'Ihe lines terminating -in arrows indicate controls exercised in the direction' of thearrow. Two

arrows with their points in contact indicate a normally operative transmission path which is broken under control of the arrow pointing toward the meetingpoint of the two arrows in contact. With this explanation in mind the general transmission features will now be outlined.

Considering speech transmitted from thevwest substation originating in the microphone M, this is amplied by the microphone amplifier MA and is passed through a volume control VL which has no eil'ect on speech of normal volume but prevents transmission of high energy peaks of speech. yAt the west toll office the speech after ampliiicationpasses over the line through the normally closed contacts of the suppressor SP to the east toll oilice; Some of the speech at this point is detectedat lD and opens the contacts of the suppressor SP of the opposite line. Some of the speech energy also removes loss from the expander EXP over the control EX. Thevspeech passes through the expander and on to the east substation Where it is lheard from the output oi the loud-speaker IS. The equipment at the two toll oices is the same so that the description of the action which takes-place at either oiiice will suice for both.

Returning to the west toll oiilce, the controls are such that the outgoing speech path which was just traced through the normally closed contacts of the suppressor SP is normally under control of the receiving side, so that if speech is incoming in the receivingside provision is made for opening the suppressor SP in the transmitting side, thus preventing transmission inthe outgoing path. The west talker can, however, break I The arrangements are such, however, that re-v ceived speech of greater than a predetermined value can disable this break control by breaking the guard contacts GD, the resistance R indicating that relatively large volume is required tov accomplish this. summarizing the vabove functions, it is seen that the outgoing path is normally under the control of the received speech; that the talkers outgoing speech can make this normal control ineffective in order to enable him to break in: but that the received speech can regain control by operating the guard control.

In addition to the foregoing controls the incomingspeech as already noted removes loss from the expander EXP. The use of anexpander to switch the loss under control of incoming speech is preferably to use ot a relay or the equivalent 4which would produce sudden and very large changes in the loss. vThe operation o f such a relay is found in practice to be annoying and ldisturbing because its contacts would be operating frequently to open and close the transmission path. The expander on the other hand is so controlled that its loss is not changed instantaneously but rather at approximately syllabic rate. For example, in a typical case it requires to 30 milliseconds to remove the loss and from70 to 90 seconds to restore the loss in the expander. The characteristic of the expander is of the general type indicated by the curve included in the rectangle which represents the expander. From this curve it is seen that the ratio of output to input is much steeper over an interme- Vdiate range of volume than it is for very weak volume or very strong volume. The settings of the circuit are such that on normal received volume from the distant terminal practically all of the loss is out of the expander all of the time that the speech is incoming. In the absence of received speech, however, a suiiiciently high loss is introduced into the expander to prevent a singing condition in the system. -It is found that the use of an expander in this manner for voice operated switching gives a much more pleasant effect than the use of a relay. This feature is disclosed and claimed in my prior application referred to above. i

The suppressor control is set to be quite sensitive and is also given a considerable hang-over period in order to guard against the eect of echoes from reverberations of the conference room, which may persist for a considerable time, for example, of the order of 200 milliseconds. This means that the suppressor control is operated a large percentage of the time when thereably less sensitive than the suppressor control` relative to normal speech volume. This is satisfactory in practice since'a speaker in trying to interrupt or break in naturally raises his voice.

The guard control is provided to prevent the operation of the break control BK on echoes oi' loud speech. For example, when speech is incoming all of the loss is switched out of the expander so that if the break control were operated by an echo of the received speech to remove the loss at the suppressor point the system would be in a singing condition. Received speech of sufiicient volume'to produce such echoes, however, is strong enough to operate the guard control. This guard circuit can be set considerably less sensitive than the suppressor, for example l0 decioles less sensitive, and also will operate satisfactorily with a shorter hang-over time.

Referring to the circuits shown in Fig. 2, th west to east transmission path at the west toll ofiice has its input at i and its output at l, while the east to west transmission path at this toll omce has its input at 35 and its output at d5.

r"hle speech incoming in circuit i is amplified at 2 and is transmitted to the outgoing circuit il through transformers 3 and The transformer 3 has its secondary divided into two parts d and The upper portion d is included in a circuit which may be traced from ground at il through resistance t, winding d, resistance l@ and li and primary winding it of transformer neighborhood of 1,000A cycles.

noise.

G toground at I4 when the suppressor relay I5 is deenergized, as normally. Resistances Il, together with shunt resistance I2, form a T-network. The winding 5 is in a circuit which may be traced from ground at 8 through the normal rest contacts of guard relays 20 and 2l, tuned circuit comprising inductance I8 and capacity I9, rectifier bridge Il, winding 5 and resistance 9 to ground 8. The resistance 9 is of such value with respect to the resistance looking into the output of amplifier 2 as to make the transformer 3 operate like a hybrid coil and to effectively isolate the separate circuit branches traced above for the respective windings 4 and 5.

Suppressor relays I5 and I6 are operated under control of the receiving side of the circuit and in the absence of received speech they are in a deenergized condition. In this condition they establish a vtalking path of low vimpedance from transformer 3 through transformer 6 to the outgoing line l. When relay I5 is energized in the manner to be described. it opens the speeech transmission circuit previously traced through I winding i3, thus interrupting the outgoing talking path. This action occurs as soon as .the armature leaves its back contact. When the armature of relay I5 closes its front contact suppressor relay it becomes energized by-current from positive battery through the winding of relay l, resistance 2l, back contact of relay 23 (assuming this relay to be deenergized), front contact of relay I5 and ground. Relay l@ in operating first removes the short circuit around resistor lli, thus increasing the loss in the outgoing speech path and then upon closure of its front contact provides a short circuit across winding d and resistor l@ soas effectively to suppress outgoing, speech. The effect of these two relays operating under control of received speech is, therefore, to disable the outing speech path, corresponding to opening the contacts of 'the suppressor SP in Fig. l.

A part of the speech impressed on incoming circuit i is transmitted through the circuit traced above including winding 5. The speech waves in this branch are rectified at' il, the rectined current is smoothed by the shunt condenser Zl and operates break control relay 22. The tuning of the circuit branch it, le is chosen to 4maize this circuit especially responsive to the high energy portion of the speech band in the This has the effect of discriminating against certain types of Relay 22 in operatingcloses an energizing circuit for break relay 12d extending from positivepole of grounded battery through the winding of relay 23, resistor 25, front contact of relay 22 and ground. Relay d@ in operating breaks the previously traced energizing circuit for suppressor relay it, which releases and replaces the short circuit around resistance it. Relay 22, upon having energized as above described, interrupted, at its back contat in a `the BK control and' removing the SP control from the receiving side. y

Considering now the receiving side ot the circuit, speech waves entering the circuit 35 pass in part through the pad 38 and through the loss circuit 40 and ampliiier 44 into thel outgoing side'46. Inorder to remove loss normally present in the circuit 4I a portion of the incoming speech waves is diverted into circuit 52 and is impressed through input transformer 53 upon the input terminals of ampliiler tube 55. The

network 54 comprising series resistance and fshunt inductance is designed to `make this cir.-

cuit sensitive to the high energy portionof the speech band in the neighborhood of 1,000 .cycles to discriminate against noise. speech waves in the output of tube 55 pass through transformer 56 to the rectier 51 where they are rectified into pulsating direct current. The lter 56 suppresses the pulsations and the resulting direct current-is impressed on tube 59. The direction of the current is such as to increase the space current through the tube 59 in proportion to the amount of the rectied input. The space current of tube 59 ows in series from the plate over` conductor 50 to the mid-point of the primary winding of transformer 43 where it divides and flows through the varistors 4I and'- 42 in parallel to the mid-point of the secondary winding of transformer 39 and returns through conductor 49, resistance 64 and conductor 63 to the positive pole of plate battery I and thence through ground 60 to the cathode of tube 59. Increase of current ilow through this circuit decreases the loss inthe circuit 40 by lowering thel I ly high signal level to be impressed on circuit ,branch to operate the suppressor relays in a manner to be described and to permita lower level to be usedjor operating the expander circuit as described.

The loss circuit 40 is constructedto be accurately 'balanced throughout in order to prevent alternating components of the control waves speech path. Resistance 64 and ycondensers 65 and 66 act to illtcr out and by-pa'ss any remaining alternating current in the plate circuit of tube 59 which might produce noise in the through transmission circuit. l

Rectiiier62 affords a by-pass around the filter 59to limit the speed of the expander action for very high input levels. Excessive voltages due to such signals are not able to be applied to the -condensers of lter 58 due to the shunt action of rectier 62 where such voltages-'exceed the amplifier 44 by way of connection 41 to anadjustable -point on grid resistor 48. Adjustment of this movable contact varies the magnitude of feedback fo'r Again control purposes. Theplate circuit is provided with a condenseraround the .'I'he amplified output coil in order to attenuate above 10,000 cycles. l

Receiving branch circuit 'I5 across incoming circuit 35 leads to the control circuits for the 5 suppressorrelay I5 and guard relay 20. Circuit 15 is shown as comprising series resistances shunted by condensers which act with the mutual inductance of the input transformer 16 to tune this circuit broadly at 1,000 cycles to disl0 criminate against noise. After amplication in 36, the speech waves in this branch are applied to the coil 31 which`acts similarly to the coil 3 in the transmitting side to divide the currents into two circuits which are mutually independent l5 of each other to a high degree from `an impedance standpoint. Thisis achieved by providing a mid-tap on the secondary of coil 31 and including resistance 1I to balance the output impedance of ampliiler 36. The upper winding zoleads to rectifier circuit 69 which rectiies the speech waves supplied to it and renders them suitable for operating suppressor relay I5. Series resistance 11 and shunt capacity 13 actj as a lter to suppress the alternating current' components. The voltage across the lower half of the secondary winding of coil 31 is applied to rectifier 61 through the intermediaryl of attenuating re- -sistances 69 and 10, the latter of which is shown as adjustable. These resistances control the relative sensitivity of the energizing circuit for the guard relay 20 to which the rectified output of rectitler 61 is applied. Series resistance 3l and shunt capacity 12 act as a lter to suppress the 'alternating components. The loss pads 69 and 10 correspond to the resistance R in the simplified diagram of Fig. 1 to insure that the guard relay 20 is operated only by received speech above a predetermined magnitude.

The various switching devices are adjusted to operate in particular timed relationship and certain of them are provided with means for giving a certain amount of hang-over time as will now be described. The suppressor relay I5 operates and releases quickly, this relay being provided with no hang-over. As its armature I4 leaves its back contact (as already described) the transmitting path is immediately interrupted between winding I3 of output transformer I6 and ground. As soon as the amature of relay I6 reaches its front contact ground is supplied over and harmonics thereof from entering the through back contact and armature of relay 23 and resistance4 21 to the upper plate of condenser 26 and to a terminal of the winding of relay I6. Condenser 26 is normally charged tothe full lilvoltage of the relay operating battery from the`- for a greater portion of the time due to the delay I release characteristic provided by' condenser 28.4 For example, iwhen the armature of relay I5 leaves itsfront contact momentarily current con-N 4 tinues to ilow through the winding of relay I6 and into condenser I8 providing hang-over. If

hang-over time is again restored to relay I6. Re-

lay I6 is released only provided relay I5 releases `its armature for more than a given time interval.,`

frequencies A relay I5 reenergizes during the hang-over period, y

condenser 29 is quickly, discharged and the full .through to the outgoing circuit l.

The resistance III controlled by the back contact of 'relay' I6 is desirable in order to provide for disabling thev transmitting path into two steps. When relay I5 releasesits armature the transmitting path is partially restored but is not fully restored until relay I6 releases. This prevents a too sudden application of high energy waves when the local party breaks in with loud speech the eiect of which might be in some cases to cause false operationof the local receiving controls by a short impulse of high magnitude just traced, as soon as the armature of relay 22 When the armature of c leaves its back contact.

introducing loss into the outgoing speech path, means operated by speech energy in the outgoing speech path to disable the operation of said rst means, and means operated by speech energy only in excess of a predetermined volume in said incoming path for preventing the disabling of the operation of said first means by said second means.

2. In a telephone system, a speech transmit'- ting circuit at a terminal station, a speech receiving circuit at said station, said transmitting and receiving circuits serving as the opposite sides of a four-wire two-way conversational path, voice-operated means operating in response to received speech in the receiving circuit for inserting loss in the transmitting circuit, voiceoperated means operating in response to speech y in said transmitting circuit for disabling the first relay 22 reaches its front contact it almost immediately discharges condenser 25 which has previously been charged to approximately full l battery voltage over the circuit including the winding of relay 23 and associated resistance to the plus terminal. Relay 23 is then energized by current ow from the plus terminal through the relay winding and resistance 26 to ground over the fvront contact of relay 22. When the armature of relay 23 -reaches its front contact condenser 28 is quickly charged to full battery potential through resistance 21, thuswiping out any remaining hang-over time for relay it. Relay IB, therefore, releases very quickly and a1- lows the speech of the breaking party to get When the local speech ceases relay 22 quickly releases and relay 23 releases after a hang-over time provided by condenser 25 in the same manner as described for relay- I6 and condenser 28. If relay 22 is not reoperated Within the hang-over time of relay 23, relay 23 releases and restores the normal control circuit between relays I5 and I6.

When a suiiiciently highv level Aof received speech is present in the branch l5 the guard relay 20 is operated asv previously described. As soon as the armature of relay 20 leaves itsback contact it disables the breaking circuit by opening the energizing circuit for the break' relay 22 at a point between the rectier il and ground. l When the armature of relay 20 reaches its front' contact it discharges condenser 29 and causes operation of relay 2l. Condenser 29 operates to give a delay release action to relay 2l in the manner previously described for relays i6 and 23. Thus, if relay 20 releases and is reenergized within the hang-over time of relay 2|,Yrelay 2l remains energized and its hang-over interval is restored.

The connection of the transformer 3l to the energizing circuits for the relays i5 and 20 to function after the manner or" a hybrid coilv as above described, lpermits'sensitivity adjustments to be made/ior-'the relay 20 without affecting the sensitivity of the relay I5. l

What is claimed is:

1.'In a two-Way speech transmission system,

an routgoing speech path and an incoming. speech path. at the same station, means operated by speech energy in the incoming speech pathfor voice-operated means, and marginally operating means operated only by speech energy above a predetermined level in the receiving circuit for disabling said second-mentioned voice-operated means.

3. In a speech transmission system, an outgoing speech path and an incoming speech path at the same station, means for disabling the outgoing path under control of received speech in the incoming circuit, break means operated by speech energy on the input side of said outgoing path for making the rst-mentioned means inoperative to disable said outgoing path, and guard means operated only by received speech energy in excess of a predetermined magnitude in said incoming circuit for rendering said break means ineiective.

4. In combination, a speech sending circuit and a speech receiving circuit at the same station, means control-led by speech in the receiving circuit for increasing the gain in the receiving circuit and increasing the loss in the sending circuit, means controlled by speech energy in the sending circuit for breaking the lossincreaser control to enable .a talker at such station to transmit from the station, and marginally operating means controlled by speech only in excess of a predetermined level in the receiving circuit for disabling the breaking means.

5. In combination, anoutgoing speech path, an incoming speech path, a singing suppressor including a quick-release relay operated from the incoming circuit and a second relay operated from the iirst rel-'ay and provided with a normal hang-over interval, said second relay` holding the singing suppressor operated during vibrations of the first relay, a breaking circuit under control of the outgoing circuit, and means operated by said breaking circuit for disabling said first relay and for restoring the second relay in less time than its normal hang-over interval.

6. In combination, a speech transmitting circuit, a singing suppressor kincluding a relay for introducing loss in said circuit, a condenser con nested to the energizing winding of said relay to provide a hold-over interval therefor by ow of its charging current through the energizing Winding of the relay, a breaking circuit controlled by speech energy in the outgoing circuit for disabling said singing suppressor, and means lcontrolled by said breaking circuit for charging said condenser over a path separate from the energizing winding of said relay to reduce said hold-over interval.

7. In combination in a two-way speech transmission system a speech sending circuit includrelay when energized.

2,287,622 -ing a singing suppressor comprising a quick- 8. In a speech' transmission circuit, an input circuit therefor having two `branches .mutually conjugate with respect to each other, one of said branches forming a partv of the through transmission circuit for speech, loss elements in said one branch, and means for switching said loss elements to disable said one branch, the second of said branches including switching control means for determining the operation of said switching means.

9. In a speech transmission system, a transmitting circuit and a receiving circuit at the same station, a singing suppressor including a suppressor relay for disabling the transmitting circuit, a breaking circuit for restoring said relay, a guard relay for disabling said breaking circuit, a switchingcontrol circuit controlled by speech energy in said receiving circuit and comprising two branches mutually conjugate with respect to each other, said suppressor relay being connected to one of said branches and said guard relay being connected to said other branch,

and means in said other branch for controlling the sensitivity of said guard relay.

- 10. In a two-way telephone system, stations including terminal stations, oppositely-directed one-way transmission paths at said stations for transmitting telephone signals in opposite directions, a transmitterand a loud-speaking receiver connected respectively to the outgoing and incoming -one-way paths at said terminal stations, a transmission device in one one-way path at one or more of said stations, normally providing a singing suppression loss therein and operating in response to applied telephone signals to effectively increase the gain of said one one-way path, voice-operated means responsive to the signals in the input of said device toy insert loss in the other one-way path at said one station, break'.` means responsive to telephone signals in said other one-way path to disable said voice-operated means, and marginal means opera'ting in responsive to telephone signals in' the input of said transmission device at said one station, of a level a given amount greater than normal level to disable said break means. A

y, r s 11..The system of claim l0. in which said transmission device is an expander type singing suppressor, one such transmissiondevice be- L ing 4used at two diilerent'stations and being lconnected in opposltely-directed one-way paths at the two stations.

12. In combination in la two-way telephone system including oppositely-directed one-way transmission paths for respectively transmitting overpthe system they voicesignals of the subscribers ai; opposite terminals-thereof, one control device connected to each path, responsive to voice signal transmission therein, in the absence of` prior voice signal transmission in the other path, to disable said other path, a second control device connected to each path, responsive to voice signal transmission therein to disable saidvone control device connected to said other path, and means to allow one subscriber to gain control of the system while another subscriber isv talking and has operated. said one control device connected to the path carrying his signals, tov

obtain directional control of the system, comprising a third relatively insensitive control de,- vice connected to each path responsive to voice signal transmission therein only of a level a predetermined amount greater than average level to disable said second control device connected to the otherA path so as to enable immediate operation of saidone control deviceconnected to the iirst path to disable said other path.

13. The combination of claim 1.2, in which said second control deviceis connected to each path in front of the disabling point therein. v

14. The combination of claim 12, in which said one control device and said third control device are connectedto each path beyond the disabling point therein and said second control device is connected to each path in front of the disabling point therein.

15. In a two-way speech transmission system, an outgoing speech path and an incoming speech path' at the same station, means operated by speech energy in the incoming speed 'path for introducing loss into the outgoing Donau Miramar.. 

